1. Field of the Invention
This invention relates to an engine speed control apparatus and particularly to an engine speed control apparatus for controlling an idling speed.
2. Description of the Prior Art
An engine speed control apparatus is known which determines a control amount as follows:
A value of a state variable presenting an internal condition of an engine is estimated through a given dynamic model in accordance with input and output control amounts. An engine speed of the engine is controlled in accordance with control inputs determined on the basis of the estimated value of the state variable, and integration values of deviation of the engine speed from a target speed.
FIG. 5 is a block diagram of a prior art dynamic model of an engine. In FIG. 5, a state variable representing an internal condition of the engine is estimated in accordance with one or more control input amounts (a flow rate of the intake air Duty, ignition timing Ig, a flow rate of fuel supply, an amount of exhaust feedback, etc.) and a control output amount (engine speed Ne) through the dynamic model including transfer functions G1(s) and G2(s). The control input is determined in accordance with the estimated value of the state variable and integrated values of deviation of the engine speed from a target engine speed. Such technique is disclosed in Japanese patent application provisional publication No. 59-145338.
Generally, the ignition timing should be set within a predetermined range so that vibrations and exhaust emissions of the engine can range within a tolerance. However, in the above-mentioned engine speed control apparatus of the engine, if one of control inputs is the ignition timing, it is not always possible to set the ignition timing within the tolerance. For example, when deviation of engine speed from a target speed is large, such as a transition state, the ignition timing is set to a value out of the predetermined range mentioned-above in order to control the engine speed to the target speed. Then, though the engine enters a stationary condition, the ignition timing is retained at the value which was set during the transition state, so that the ignition timing is not set within the tolerance during the stationary condition. Thus, there is a drawback that the resultant controllability decreases, such that vibrations becomes large and emissions become large.